The invention relates to a mosquito-killing bacillus thuringiensis granular preparation and a preparation method thereof. The preparation method of the mosquito-killing bacillus thuringiensis granular preparation comprises the steps of proportioning, preparing a calcium chloride solution, preparing a floating agent, hardening and drying. According to the invention, sodium alginate is adopted as a carrier, and calcium chloride is adopted as a cross-linking agent; bacillus thuringiensis raw powder added is an effective active ingredient and has the activity of efficiently killing mosquitoes; the granular preparation achieves the floatation slow-release effect by virtue of hollow glass beads, the ultraviolet radiation is reduced, and the lasting period is prolonged; melanin added is an ultraviolet protection additive, and enhances the ultraviolet radiation resistance of the bacillus thuringiensis. The product prepared by the invention is a solid, and is reasonable in formula and high in insecticidal toxicity; during the use, the mosquito-killing bacillus thuringiensis granular preparation is only required to be spread on water surfaces where mosquitoes breed, so that the control efficiency is excellent, and high convenience is brought to development and promotion as well as popularization and application.